Commercial aircraft have typically used bleed air from their turbine propulsion engines to provide motive power for subsystems such as environmental controls and brakes. However, this use of bleed air reduces efficiency of the turbine engines. Moreover, ducts, valves and controls for supplying the bleed air to the subsystems add weight to the aircraft.
More advanced commercial airliners may use electricity to power certain subsystems that, in the past, were powered by bleed air. The electrical power is supplied from generators driven by the propulsion engines. Backup electrical power is available from generators driven by an on-board auxiliary power unit (APU). Additional backup electrical power is available from a ram air turbine.
Rechargeable batteries provide yet another source of backup electrical power. However, nickel-cadmium (Ni—Cd) batteries, which were used on earlier commercial aircraft, aren't feasible for providing the backup power to certain subsystems.
Lithium-ion (Li-ion) batteries (LIBs) are the rechargeable batteries of choice. Lithium-ion batteries such as lithium cobalt oxide batteries have lower weight and higher energy density than Ni—Cd batteries. They have no memory degradation.
Certain lithium-ion batteries have longstanding issues with thermal runaway. As used herein, thermal runaway means a situation where an increase in temperature causes a further increase in temperature that may lead to decreased efficiency. For example, heat generated from an exothermic chemical reaction may increase the rate of the chemical reaction. Designers of complex systems may address such inefficiencies in various ways.